Methods of encoding a television signal have been proposed in which, with a view to narrowing a transmission band, the average bit number per picture element or a sampling frequency have been reduced.
In an encoding method for reducing a sampling frequency, it has been proposed to thin the picture data to one half by subsampling and by transmitting subsampling points and flags for indicating the positions of the subsampling points to be used in the interpolation (i.e., for indicating which of the data of the subsampling points lying above and below or to the left and right of interpolation points should be employed).
A DPCM (differential PCM) encoding method has been known for reducing the average bit number per picture element. The DPCM method pays attention to the facts that there is a high correlation between picture elements of a television signal and that the difference between adjacent picture elements is small, and transmits this difference signal through quantization.
Another encoding method of reducing the average bit number per picture element involves dividing one field picture into small blocks and then transmitting an average value, a standard deviation and an encoding code of one bit in one-to-one correspondence with each picture element.
In the encoding method for reducing a sampling frequency through the use of a subsampling technique, there is the possibility that folded distortion may occur, because the sampling frequency is reduced to one half.
The DPCM method is burdened with the problem that some error propagates toward subsequent encoding.
The method for performing the encoding in a block unit has the disadvantage that block distortion occurs at the boundary between blocks.
The present applicant has proposed a highly efficient coding apparatus for obtaining a dynamic range defined by a maximum value and a minimum value of plural picture elements contained in a two-dimensional block and performing encoding adaptive to the dynamic range, for example as described in copending U.S. patent application Ser. No. 06/809,742 having a common assignee herewith. In that application, deterioration of the picture quality of a reproduced picture is substantially avoided, because a reversible encoding technique is employed. However, when blocks with large dynamic ranges appear continuously, the bit number for encoding increases to exceed the transmission-allowable capacity. As a result, the transmission at a predetermined rate cannot be achieved.